what represents the best performance? if thats your question then its the 6950 crossfire, though if the 580 is cheaper it would be my preferred option as it's not too far behind and its a single GPU solution and it will give you near enough the same performance without the hassle of a dual GPU setup.

Well single card is always generally preferable but with the scaling on the 6800 series combined with the fairly significant lower price I say definitely the 6850's. I suppose I should note I just bought two myself...

in terms of performance if hes at 1920x1200 he should with the newer drivers AND if hes on an intel core i5 or i7 be about 35% faster with the dual cards any lower resolution the 580 keeps up and if hes on an AMD rig again dual cards of the 6900s are a waste there i should know i own 2 x 6970s

nope 6800s scale fine way better then the 5800s just seems that what cadaveca pointed out was fairly sorta accurate after 1600 shaders amd dosent have the grunt to feed them so 960x2 is fine 1120 is fine its not a huge deal but for me 1536 x 2 = well double what the cpu can feed its why most games i play only show 30-40% usage on both gpus...

Ya I know that's why i went 6850 instead of buying another 5850. I was asking about scaling in terms of 6900 vs 6800. Figure it's similar if not equal but haven't chekced the numbers for myself. I'm not really a numbers kinda guy ;P

in daul card configs 6950s are on part with dual 570s and dual 6970s are on par with dual 580s so grabbing 2 580s is far more expensive for comparable performance as the 2 6950s if overclocked will be only 5%-8% slower then dual 580s at a near $400 cheaper comparatively. That said depending on Resolution and CPU id say 580 as well but if on i7 and at 1920x1200 or 1080p then dual 6950s

scaling on 6800 on i7 is around 70-80% scaling on 6900s if on i7 is around 70-100% depending on the game or senario

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Ok so 69xx is a bit better scaling-wise but the whole 6xxx series is the best yet on the market in terms of getting the most out of your second card. I thought I saw a post here that the average 6850 was 87% but even topping out at 80 is great.

but on i7 scaling is great regarless AMD and Nvidia dual card configs scale about the same now

even the 5800 scale good on i7 but yea 6800s scale better and 6900 its just driver flukes single cards seem underpowered dual cards scale better then 100% which makes no sense

that said id really need to know his full system specs but more often then not a single 580 would be better overall if he dosent feel like changing drivers often as the performance will be good in almost every situation where as any dual card setup there hit or miss depending on target

example Bad Company 2 Multi player Crossfire scaling for me at least is 8-18% single player its 40-60% obviously thats some huge discrepency and something to keep in mind

I mostly game Starcraft 2 for the moment. Do you know of any reviews comparing 6950 vs 580? What I'm really looking for is worst case scenarios, that is 2 x 200 supply battles and even 6 x 200 supply battles. Reviewers typically monitor FPS in standard games which is pretty close to worthless. Now what is worth to know is how low the FPS can drop, 20 vs 30 is a biggie.